Carotenoids in photosynthesis: Protection of D1 degradation in the light

Photosynthesis has been determined with mutants of Anacystis which form different amounts of carotenoids. With these cultures a highly significant correlation between photosynthetic oxygen evolution and the amounts of synthesized carotenoids was observed. In addition, the influence of carotenoids on light-dependent degradation of thylakoid proteins was investigated with Scenedesmus cultures grown in darkness in the presence of norflurazon, an inhibitor of carotenoid biosynthesis. Pre-illumination of cells resulted in decrease of photosynthetic activity accompanied by loss of the D1 protein. This effect is dependent on the length of illumination, and the light intensity, and increased when carotenoid content was lowered during previous growth of the norflurazon-treated cultures.Abbreviations BSA bovine serum albumin - D1 32 kDa Q_B-binding protein - EDTA ethylenediaminetetraacetic acid - LHCII light-harvesting complex II - PMSF phenylmethylsulfonyl fluoride - PS photosystem - tricine N-[tris(hydroxymethyl)methyl] glycine     

DNA binding factor GT-2 from Arabidopsis

Complementary DNA clones encoding a DNA-binding factor have been obtained from Arabidopsis by DNA hybridization with a GT-2 factor cDNA clone from rice. The GT-2 gene appears to be present as a single copy in the Arabidopsis genome and is transcribed as a 2.1 kb mRNA which is not light-regulated. The longest open reading frame in the sequenced clones predicts a protein of 65 kDa, beginning with the first in-frame methionine. The protein contains basic, acidic, and proline/glutamine-rich motifs and has significant amino acid sequence homology to the rice GT-2 factor, including three regions of 50–75 amino acids each of greater than 60% identity. Two of these regions are predicted to form similar trihelix structures postulated to be involved in selective binding to specific variations of a GT-box motif DNA sequence found in the promoter regions of several plant genes. Except for weak similarity to a tobacco GT-box binding factor, GT-1a/B2F, Arabidopsis GT-2 has no similarity to other sequences in the databases. DNA-binding studies show that Arabidopsis GT-2 has binding characteristics similar to those of the rice GT-2 factor, but dissimilar to those of the tobacco GT-1a/B2F factor. The data indicate that a DNA-binding factor containing domains of similar structure and target-sequence specificity has been conserved between monocots and dicots.     

Endonuclease-sensitive DNA modifications induced by acetone and acetophenone as photosensitizers.

Repair endonucleases, viz. endonuclease III, formamidopyrimidine-DNA glycosylase (FPG protein), endonuclease IV, exonuclease III and UV endonuclease, were used to analyse the modifications induced in bacteriophage PM2 DNA by 333 nm laser irradiation in the presence of acetone or acetophenone. In addition to pyrimidine dimers sensitive to UV endonuclease, 5,6-dihydropyrimidines (sensitive to endonuclease III) and base modifications sensitive to FPG protein were generated. The level of the last in the case of acetone was 50% and in the case of acetophenone 9% of the level of pyrimidine dimers. HPLC analysis of the bases excised by FPG protein revealed that least some of them were 8-hydroxyguanine (7,8-dihydro-8-oxoguanine). In the damage induced by direct excitation of DNA at 254 nm, which was analysed for comparison, the number of FPG protein-sensitive base modifications was only 0.6% of that of the pyrimidine dimers. Mechanistic studies demonstrated that the formation of FPG protein-sensitive modifications did not involve singlet oxygen, as the damage was not increased in D2O as solvent. Hydroxyl radicals, superoxide and H2O2 were also not involved, since the relative number of single strand breaks and of sites of base loss (AP sites) was much lower than in the case of DNA damage induced by hydroxyl radicals and since the presence of SOD or catalase had no effect on the extent of the damage. However, the mechanism did involve an intermediate that was much more efficiently quenched by azide ions than the triplet excited carbonyl compounds and which was possibly a purine radical. Together, the data indicate that excited triplet carbonyl compounds react with DNA not only by triplet-triplet energy transfer yielding pyrimidine dimers, but also by electron transfer yielding preferentially base modifications sensitive to FPG protein, which include 8-hydroxyguanine.     

Evidence for the Predator Attraction Hypothesis in an amphibian predator–prey system

Many species possess damage-released chemical alarm cues that function in alerting nearby individuals to a predator attack. One hypothesis for the evolution and/or maintenance of such cues is the Predator Attraction Hypothesis, where predators, rather than prey, are the “intended” recipients of these cues. If a predator attack attracts additional predators, these secondary predators might interfere with the predation event, providing the prey with a better chance to escape. In this study, we conducted two experiments to explore this hypothesis in an amphibian predator/prey system. In Experiment 1, we found that tiger salamanders (Ambystoma mavortium) showed a foraging attraction to chemical cues from wood frog (Lithobates sylvaticus) tadpoles. Salamanders that were experienced with tadpole prey, in particular, were strongly attracted to tadpole alarm cues. In Experiment 2, we observed experimental encounters between a tadpole and either one or two salamanders. The presence of the second predator caused salamanders to increase attack speed at the cost of decreased attack accuracy (i.e., increasing the probability that the tadpole would escape attacks). We also found that the mere presence of visual and chemical cues from a second predator did not affect this speed/accuracy trade-off but did cause enough of a distraction to increase tadpole survival. Thus, our findings are consistent with the Predator Attraction Hypothesis for the evolution and/or maintenance of alarm cues.     

Three-dimensional species distribution modelling reveals the realized spatial niche for coral recruitment on contemporary Caribbean reefs

The three-dimensional structure of habitats is a critical component of species' niches driving coexistence in species-rich ecosystems. However, its influence on structuring and partitioning recruitment niches has not been widely addressed. We developed a new method to combine species distribution modelling and structure from motion, and characterized three-dimensional recruitment niches of two ecosystem engineers on Caribbean coral reefs, scleractinian corals and gorgonians. Fine-scale roughness was the most important predictor of suitable habitat for both taxa, and their niches largely overlapped, primarily due to scleractinians' broader niche breadth. Crevices and holes at mm scales on calcareous rock with low coral cover were more suitable for octocorals than for scleractinian recruits, suggesting that the decline in scleractinian corals is facilitating the recruitment of octocorals on contemporary Caribbean reefs. However, the relative abundances of the taxa were independent of the amount of suitable habitat on the reef, emphasizing that niche processes alone do not predict recruitment rates.     

Insertional mutagenesis to isolate acetate-requiring mutants in Chlamydomonas reinhardtii

Abstract An arg 7 mutant of the green alga Chlamydomonas reinhardtii was transformed with pARG7.8, a plasmid bearing the wild-type ARG 7 gene. Out of 4100 arg⁺ transformants selected on an arginine-free medium supplemented with acetate, nine failed to grow on acetate-free medium (ac⁻ mutants). The results of the genetic and molecular analysis of several ac⁻ mutants are in agreement with the hypothesis that they originated from insertion of the incoming plasmid into the nuclear genome. These mutants should constitute valuable tools for isolating the corresponding wild-type genes after plasmid rescue into Escherichia coli .     

Biochemical changes in human cervical connective tissue after intracervical application of prostaglandin E2

Cervical biopsies were taken during the first trimester from primigravidae and plurigravidae at different time points after intracervical application of prostaglandin E₂-gel. Collagenase activity was determined by a highly specific technique using native, triple helical collagen as substrate. Elastase-_α1-proteinase-inhibitor complex (elastase) was measured by a commercially available assay, and glycosaminoglycan (GAG) analyses were performed as described by Greiling et al. (5, 6). The maximum activity of collagenase was found 2 hours after PGE₂ application in plurigravidae and 4 hours after application in primigravidae. Elastase activity rose nearly 7-fold to maximum values 4 hours after PGE₂ application. The total GAG concentrations and the dermatan sulfate concentrations increased by approximately 10 %, while the hyaluronic acid concentrations were found to be elevated significantly by nearly 50 % in the PGE₂-primed cervices.We conclude that a time-dependent enzymatic collagen degradation by collagenases and other proteinases and an increase in hyaluronic acid concentrations are the significant biochemical events underlying PG-induced cervical ripening.     

Reduction in animal abundance and oxygen availability during and after the end-Triassic mass extinction

The end-Triassic biodiversity crisis was one of the most severe mass extinctions in the history of animal life. However, the extent to which the loss of taxonomic diversity was coupled with a reduction in organismal abundance remains to be quantified. Further, the temporal relationship between organismal abundance and local marine redox conditions is lacking in carbonate sections. To address these questions, we measured skeletal grain abundance in shallow-marine limestones by point counting 293 thin sections from four stratigraphic sections across the Triassic/Jurassic boundary in the Lombardy Basin and Apennine Platform of western Tethys. Skeletal abundance decreased abruptly across the Triassic/Jurassic boundary in all stratigraphic sections. The abundance of skeletal organisms remained low throughout the lower-middle Hettangian strata and began to rebound during the late Hettangian and early Sinemurian. A two-way ANOVA indicates that sample age (p < .01, η² = 0.30) explains more of the variation in skeletal abundance than the depositional environment or paleobathymetry (p < .01, η² = 0.15). Measured I/Ca ratios, a proxy for local shallow-marine redox conditions, show this same pattern with the lowest I/Ca ratios occurring in the early Hettangian. The close correspondence between oceanic water column oxygen levels and skeletal abundance indicates a connection between redox conditions and benthic organismal abundance across the Triassic/Jurassic boundary. These findings indicate that the end-Triassic mass extinction reduced not only the biodiversity but also the carrying capacity for skeletal organisms in early Hettangian ecosystems, adding to evidence that mass extinction of species generally leads to mass rarity among survivors.